A read-only memory (ROM) is a type of memory in which data can be permanently stored, for example, by blowing metallic links during programming thereof. Such type of memory can be conventionally accessed at specific locations to read the programmed contents thereof. A much more versatile type of ROM is the electrically programmable read-only memory (EPROM) which can be electrically programmed to store data. Some versions of these devices store data permanently; others, however, can be erased with ultraviolet light or an electrical current.
The construction of an EPROM cell is similar to that of a field effect transistor, but additionally includes a floating gate between the gate conductor and the conduction channel of the transistor. During programming to store a desired logic state, electrons flowing in the conduction channel ar attracted and trapped in the floating gate. This condition increases the threshold voltage of the transistor, thereby rendering it cutoff in response to normal read operation voltages. Hence, during reading of a transistor cell so programmed, the transistor will remain nonconductive and thus represent a high impedance between the source and drain. On the other hand, cells which have not been programmed will remain conductive in response to normal read operation voltages. In this manner, the memory can be programmed.
Source side injection (SSI) is a very efficient mechanism for generating hot carriers and a significant gate current. A SSI FAMOS (Floating gate Avalanche Metal Oxide Semiconductor) cell generally consists of a highly resistive region at the source whose conductivity is not easily modulated by the control gate voltage. Under high gate bias, a high channel electric field region is created at the source, along with a high vertical field there due to high gate-to-source voltage.
An ideal SSI cell should have the properties of significant gate current at low drain bias and high read current. These properties, however, are not complementary to each other. In other words, enhancing one property will degrade the other one. As such, a cell is presently desired in which each of these properties can be optimized.
In an ideal non-volatile memory cell, there would be no current flow between the source and drain during programming. In present cells, however, a current does flow from the source to drain during programming thereby dissipating power. Therefore, it is also desirable for a cell which reduces the amount of source/drain current during programming.
Accordingly, improvements which overcome any or all of the problems are presently desirable.